Many olefin polymerization catalysts are known, including Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity and are generally poor at incorporating α-olefin comonomers. The large variety of active sites in Ziegler-Natta catalysts makes it difficult to control polymer architecture. To improve polymer properties, single-site catalysts, in particular metallocenes are beginning to replace Ziegler-Natta catalysts.
Slurry reactors are in widespread use for production of polyethylene homo- and copolymers. Slurry reactors include stirred-tank reactors and water-jacketed tubular reactors arranged in a series of continuous horizontal or vertical loops. A “slurry solvent” in which polyethylene has low solubility constitutes the continuous phase in such reactors. The slurry is intensely stirred in a continuous stirred-tank reactor or series of reactors or, in the case of slurry loop reactors, is driven around the loop at relatively high speed by one or more rather massive pumps. Ethylene, supported catalyst, comonomers, and processing additives are injected into the reactor where polymerization takes place, creating a slurry of polyethylene in solvent.
Multi-zone slurry polymerizations of ethylene with Ziegler-Natta catalysts are known. For example, U.S. Pat. No 4,357,448 discloses a two-step process for polymerizing ethylene in the presence of a Ziegler-Natta catalyst in combination with a reaction product of a titanium or vanadium halogen-containing compound with a first reaction product of a Grignard reagent with a hydropolysiloxane. U.S. Pat. No. 6,486,270 discloses a process to polymerize ethylene with a C3-C10 α-olefin in the presence of high levels of hydrogen to make polyethylene with a density of 0.92 to 0.94 g/cm3 with multiple reaction zones using a Ziegler-Natta catalyst.
There has been some use of single-site catalysts in two reaction zones. U.S. Pat. No. 6,566,450, for example, discloses a process using bis-indenyl complexes to produce polyethylene useful as pipe resin. The polyethylene has a bimodal molecular weight distribution and a density from 0.95 to 0.96 g/cm3. U.S. Pat. No. 7,423,098 discloses a multi-zone slurry process that copolymerizes ethylene with a C6-C10 α-olefin in each of the zones to produce polyethylene with a bimodal molecular weight distribution. Both make a copolymer, not a homopolymer in the first reactor and there is no teaching about resin long-chain branching or about the environmental stress crack resistance (ESCR) properties of pipe or molded products.
U.S. Pat. Nos. 6,232,260 and 6,451,724 disclose the use of transition metal catalysts based upon indenoindolyl ligands. Indenoindolyl catalysts are remarkably versatile because substituent effects and bridging changes can often be exploited to provide polymers with tailored physical or mechanical properties. Unbridged indenoindolyl complexes (as exemplified in the '260 patent) usually provide favorable activity although they sometimes fail to provide polymers having high enough molecular weights. Bridged indenoindolyl complexes (as taught, e.g., in U.S. Pat. No 6,908,972) readily copolymerize α-olefins and provide polymers with high molecular weight and varying levels of long-chain branching. For a discussion of long-chain branching in polyethylene, see Macromolecules 39 (2006) 1474 and references cited therein. U.S. Pat. Appl. Publ. Nos. 2009/0062487 and 2009/0062490 disclose a slurry process to make an ethylene copolymer with certain bridged indenoindolyl complexes including the complexes useful in the current inventive process. There is no teaching about making a homopolymer in a first reactor followed by a copolymer in a second reactor, and there is no disclosure of pipe or molded articles having improved ESCR properties. Polymers having Mw/Mn values greater than 20 are not taught.
Despite the considerable experience with single-site catalysts generally and indenoindolyl catalysts in particular, there is a need for improvement. Often, polyethylene with good processability gives pipe or molded products with poor environmental stress crack resistance. A valuable process would provide polyethylene that can be readily processed in demanding applications such as pipe or large-part blow molding to provide products with good ESCR properties.